Fix html_last_updated_fmt for Python3.
Fix html_last_updated_fmt for Python3.

html_last_updated_fmt option is interpreted as a
byte string in python3, causing Sphinx build to break.
This patch makes it utf-8 string.

Change-Id: I86b87e5672ad367b07d4f1000c5af8649ffcd0d2
Closes-Bug:#1693670

openstack/python-senlinclient,stackforge/python-senlinclient,openstack/python-senlinclient,stackforge/python-senlinclient